Light affects parental provisioning behaviour in a cavity‐nesting Passerine

Nocturnal bird species possess special adaptations to maximise visual efficiency under low light levels. However, some typically diurnal species also experience low‐light environments. For example, cavity‐nesting Passerines raise broods in dark cavities and search for food in light‐abundant surroundings. It is not clear whether they possess special adaptations for low light vision or breed in cavities at the expense of impaired parental care. In this study, we tested whether light conditions affect the provisioning efficiency of great tits. We experimentally tested how the level of natural and artificially increased illumination inside nest boxes affects parental feeding duration, frequency and timing. We monitored 15‐h of provisioning activity of great tit parents when nestlings were day seven post hatch. We used traditional ‘dark’ nest boxes and ‘bright’ nest boxes with increased illumination obtained by using semi‐transparent plastic windows. The duration of single feedings were, on average, shorter in brightened nest boxes compared to dark ones. This difference tended to be higher early in the morning and in the evening, when the illumination in dark nest boxes was the lowest. Nest box type, however, did not influence feeding frequency or times of the onset and the end of feeding. Our findings provide new evidence for impaired efficiency of parental care due to lowered light conditions. Further research is needed to test whether prolonged feeding duration has negative effects on adult time budgets and nestling energy expenditures.     

Parametric and nonparametric entrainment of circadian locomotor rhythm in the Japanese honeybee Apis cerana japonica

The circadian rhythm of locomotor activity in the Japanese honeybee Apis cerana japonica was studied to determine the involvement of parametric and/or nonparametric entrainment. The rhythm was entrained to a skeleton photoperiod in which a 1-h first light pulse was imposed in the morning along with a second light pulse in the evening, as well as to a complete photoperiodic regime (LD 12:12). However, the timing of peak activity relative to the lights-off in the evening in the skeleton photoperiod was earlier than that in the complete photoperiod. A single daily light pulse in the evening entrained the rhythm, whereas a daily light pulse in the morning allowed free-running as in constant darkness. The free-running period (τ) of locomotor activity in constant light became longer as the light intensity increased. A Winfree's type I phase response curve of the locomotor activity rhythm was obtained using a single 1-h light pulse. The results suggest that both parametric and nonparametric entrainment are involved in the circadian rhythm of individual locomotor activity in this honeybee.     

Daily flight activity of moths in the continuous daylight of the arctic summer

The daily onset of flight and the lime of activity of moths were studied at an arctic and a subarctic locality in midsummer under natural light conditions but with all other factors kept constant. The daily variation of light intensity is much smaller than at temperate latitudes, and the illumination is continuously at levels that normally inhibit the activity of moths. In spite of this, the investigated species showed a distinct daily periodicity with the activity occurring at about the same time of day as in the same or related species from lower latitudes. It is suggested that this is the result of a process of acclimatization to high light intensities.
A number of species show a diurnal activity pattern, and it is suggested that this is connected with their high temperature requirements. In the field, Anarta zetterstedti Staud. thermoregulates by settling frequently on dark rocks which can reach high temperatures when the sun is shining. This species can not fly unless its body temperature is above 30°C and it prefers an ambient temperature of about 36°C.     

Crepuscular activity of adult Sericesthis geminata (Coleoptera: Scarabaeidae): Influence of circadian rhythmicity and light intensity

Subterranean Sericesthis geminata (Boisduval) beetles emerge from the soil daily during the flight season. Emergence and flight are initiated within 15 min after sunset. The beetles are active above the soil surface as the intensity of illumination decreases from 240 lux to 0.7 lux.

If young adult beetles still in their pupal cells in the soil are placed under constant, low‐intensity illumination, they first emerge at any time of day, showing no cyclical diurnal activity pattern. When such beetles are exposed to diurnal fluctuations in illuminance, the first emergence and flight occur at dusk. These beetles subsequently show a diurnal rhythm in their activity, even when exposed to constant environmental conditions. This endogenous activity cycle has a period of about 22 h, and is synchronised with diurnal fluctuations in light intensity. It is reset in response to a change in the photoperiod.

The data suggest that, after initial emergence from the soil, daily crepuscular activity is initiated by an endogenously controlled activity rhythm which causes the beetles to burrow to the soil surface shortly before dusk. At the soil surface they are exposed to fluctuations in the duration and intensity of illumination which may reset the endogenous rhythm, affecting the time of subsequent emergence.     

BIRDS OF THE UNDERSTORY OF LAKE-SHORE FORESTS ON THE ENTEBBE PENINSULA, UGANDA

Monthly mist-netting of low-flying forest birds was conducted in three lake-shore forests in southern Uganda from September 1970 to July 1972. It was found that the edge of the forest formed a distinct ecological barrier that was rarely crossed by species on either side of it. Catches were concentrated in the hours of daylight, in two peaks, a morning peak at 09.00 hrs and an afternoon peak at 16.00 hrs (sun-time) for all birds combined, although there were slight species differences. Most bird species were found to be sedentary in a fixed home-range, and covered lateral distances of up to c. 300 m. No movement between the different forests was recorded.
The most common birds such as bulbuls were often caught at heights between 0·6 and 1·8 m above the ground, but the upper limit of their vertical range was not determined. It is suggested that the bi-modality of flight activity may be intrinsic, even though it may be correlated with such extrinsic factors as light and radiation. On a month to month basis, the numbers of birds caught reflected breeding and moult activity. This was best shown by the Pygmy Kingfisher, taken in higher numbers towards the end of the first rainy season when a majority of the birds caught were young and many birds were moulting the remiges. On the other hand higher numbers of Olive Sunbird were found during the second rainy season, again at a time corresponding to a general moulting of remiges. The bulbuls showed fluctuations in numbers caught that were matched by an equally fluctuating pattern of moult. The role of food as a possible limiting factor in bird breeding is discussed.     

Effects of Natural and Artificial Light in Arctic Latitudes on Long- and Short-day Plants as Revealed by Growth Analysis

The effects on growth and flowering of two short-day and twolong-day plants when grown under different conditions of illuminationare described. The plants fully investigated were Kalanchoeblossfeldiana and Xanthium pennsylvanicum and the annual varietiesof Hyoscyamus niger and Beta vulgaris. Wintex barley, Iberisumbellata, and tomato were also grown in some selected treatments.The conditions investigated comprised continuous full daylight(24 hours), full daylight for the whole of the daily photoperiodand full daylight for half the photoperiods, the other halfconsisting of either daylight reduced by shading or light fromincandescent lamps or fluorescent tubes (daylight-matching type),all of the same low intensity. Two lengths of photoperiod wereused for each species, one nearly optimal for flowering, theother closer to the critical day-length; and the order of thelow and high light treatments was varied. These factors werecombined factorially. Data were collected (or derived) for the following characteristics,though not always for all the species grown: dry weights, leafareas, heights, water contents, epidermal cell sizes, net assimilationrates, times to flowering, leaf-number increments until flowering,numbers of inflorescences, stomatal apertures, and leaf postures. Among other effects, the data revealed that in all four speciesinvestigated the adverse effects on over-all growth to be expectedfrom reduction of the daily photoperiod or of the intensityof illumination are in fact minimized. This compensation waseffected mainly by large increases in leaf areas, even thoughin all cases half the daily photoperiod consisted of full daylight.There are indications that increased efficiencies (net assimilationrates) may also have been involved. The leaf-area increasesappear to have been due to increased cell size rather than cellnumber and a close positive correlation with water content wasfound. The most striking among the effects on flowering was the failureof sugar-beet to bolt when half of its photoperiod (totals of20 and 14 hours) consisted of light from fluorescent lamps.The flowering of barley and Hyoscyamus was also delayed considerablyunder these conditions. The deficiency of red in the spectrumof the fluorescent light is believed to have been the cause.By contrast, the flowering of Iberis, a crucifer, was not affected.     

The importance of illumination level in the daily onset of flight activity in nocturnal moths

ABSTRACT. The daily onset of flight activity in nocturnal moths from Denmark and Florida was investigated in natural and experimental reductions of illumination. The daily cycle of illumination is described, especially the variation of the rate of change during the evening. Different species commence activity ('onset') at different mean light intensities. These means occur around the termination of twilight, at between 0 and –2.50 log lux. In a given species the illumination at mean onset is constant, and the dispersion of onsets in the population is constant with respect to the illumination. As a consequence, the dispersion with respect to time increases, the longer the duration of twilight. The temporal dispersion, therefore, depends upon season and latitude. A model describing the number of onsets in relation to the decrease of illumination is presented. At the physiological level it is proposed that a preparatory process precedes the actual onset of activity. This is observed as a latency between the light change and the activity onset. It is presumed to represent a change, from an inhibiting to a disinhibiting output from a controlling CNS centre, brought about by the reduction in illumination increasing the sensitivity of the centre to a 'sensitizer'. This might represent the Zeitgeber effect of illumination reduction. The results indicate that during a gradual decrease of illumination, the latency process is initiated by a certain low illumination and that it proceeds further, the lower the illumination. The faster the rate of decrease, the faster is the rate of the process, so that it is completed at about the same time as a threshold illumination for overt activity is reached, irrespective of the rate of decrease. However, at very fast reductions of illumination, the physiological process cannot keep pace with the reduction, and activity onset occurs at a lower illumination than normal.     

A Stochastic Burst Follows the Periodic Morning Peak in Individual Drosophila Locomotion

Coupling between cyclically varying external light and an endogenous biochemical oscillator known as the circadian clock, modulates a rhythmic pattern with two prominent peaks in the locomotion of Drosophila melanogaster. A morning peak appears around the time lights turn on and an evening peak appears just before lights turn off. The close association between the peaks and the external 12:12 hour light/dark photoperiod means that respective morning and evening peaks of individual flies are well-synchronized in time and, consequently, feature prominently in population-averaged data. Here, we report on a brief but strong stochastic burst in fly activity that, in contrast to morning and evening peaks, is detectable only in single fly recordings. This burst was observed across 3 wild-type strains of Drosophila melanogaster. In a single fly recording, the burst is likely to appear once randomly within 0.5–5 hours after lights turn on, last for only 2–3 minutes and yet show 5 times greater activity compared to the maximum of morning peak with data binned in 3 minutes. Owing to its variable timing and short duration, the burst is virtually undetectable in population-averaged data. We use a locally-built illumination system to study the burst and find that its incidence in a population correlates with light intensity, with ~85% of control flies showing the behavior at 8000 lux (1942 μW/cm²). Consistent with that finding, several mutant flies with impaired vision show substantially reduced frequency of the burst. Additionally, we find that genetic ablation of the clock has insignificant effect on burst frequency. Together, these data suggest that the pronounced burst is likely generated by a light-activated circuit that is independent of the circadian clock.     

Feeding effects on daily locomotor activity in Carabus auronitens

Effects of food supply on daily periodic locomotor activity in the ground beetle Carabus auronitens were recorded in the laboratory with the animals exposed to natural daylight and photoperiod. Inspection of individual actograms exhibits that in most cases feeding leads to strongly reduced and more or less irregularly distributed activity for a period of up to 4 days. In terms of quantitative parameters, the animals respond to food supply by an immediate reduction of mean amounts of daily activity. This is achieved through the reduction of duration (as a consequence of reduced burstlengths instead of burstnumber) and, less distinctly, the reduction of intensity. The initial levels of parameters are reattained after 4 days. Direct comparison by means of standardized (i.e. level-independent) relative parameter functions demonstrates close concordance between the amount and duration of daily activity; however, distinct synchronous change of intensity is apparent as a consequence of food supply only. This means that any variation of activity amounts is due to changing duration rather than to intensity variation. In analogy to the feeding-dependent parameter dynamics, also the proportion of animals with strong synchronization to the natural light/dark cycle is considerably reduced by food supply. The feeding-dependent reduction of activity is paralleled by temporarily reduced activity of proteolytic enzymes in the beetles. It is argued that, for the purpose of energy saving, the animals will remain inactive as long as reduced enzyme activity inhibits further consumption and digestion of food.Abbreviations DD constant darkness - LD light/dark cycle - nLD natural light/dark cycle - endogenous periodlength     

Movement and migration of diatoms on sand grains

The motile species of diatoms in a largely non-motile association on grains of sand travel to the surface of the sand when buried, although they move more slowly and less often than species living in silt, and ascend each grain in turn. Diatoms on sand grains from a freshwater lake are found to migrate daily to and from the surface grains, whether in a natural daylight régime or in continuous darkness. They do not leave the surface when exposed to 750 lux of constant illumination although they will at 75 lux. Their movements help to keep them in the photic zone during the day.     

青蛙光敏感松果体神经节细胞自发放电的昼夜节律变化

为了研究青蛙松果体的昼夜节律,本实验采用细胞外连续记录,研究了松果体内对光产生抑制反应的光敏神经节细胞放电的昼夜节律变化.结果表明(1)所有表现自发放电的细胞,其放电频率介于1-6Hz之间,脉冲发放特征有规则、不规则(或波动)和阵发等方式;(2)无论在持续黑暗(DD)、持续光照(LL)或在模拟自然光照周期(L-D-L)任何一种条件下所进行的昼夜连续记录都显示:有些细胞在白天放电频率低,在夜间放电增强;而另一些细胞则在整个记录过程中,其脉冲频率基本上保持不变.当用与昼夜光照颠倒的周期(D-L-D)作实验时,所有被检测的细胞都反映出夜间的放电活性受到了不同程度的抑制.     

Effects of housing conditions and season on the activity rhythm of spider monkeys (Ateles geoffroyi) kept under natural conditions within their distributional range in Central Mexico

The timing and pattern of mammalian behavioral activities are regulated by an evolutionary optimized interplay of the genetically based biological (circadian) clock located in the brain’s suprachiasmatic nuclei and direct responses to environmental factors that superimpose and thus mask the clock-mediated effects, the most important of which is the photically induced phase-setting (synchronization) of the circadian rhythmicity to the 24-hour solar day. In wild and captive animals living under the natural conditions prevailing in their habitat, to date, only a few attempts have been made to analyze the role of these two regulatory mechanisms in the species’ adaptation to the time structure prevailing in their habitat. We studied the impact of housing conditions and season on the daily timing and pattern of activity in Mexican spider monkeys (Ateles geoffroyi). To this end, we carried out long-term activity recordings with Actiwatch® AW4 accelerometer/data-logger devices in 11 adult Ateles living under identical natural lighting and climatic conditions in either a large wire netting cage or a 0.25?ha forest enclosure in the primatological field station of Veracruz State University near Catemaco, Mexico. In a gravid female in the forest enclosure, we obtained first-hand information on the effect of late pregnancy and parturition on the monkey’s activity rhythm. The Ateles behaved strictly diurnal and undertook about 90% of daily total activity during this activity time. Due to a higher second activity peak in late afternoon, the bimodal activity pattern was more pronounced in monkeys living in the forest enclosure. Although the spider monkeys kept there had an earlier activity onset and morning activity peak than their conspecifics in the cage, no consistent differences were found in the parameters characterizing the phase-setting of the circadian system to the environmental 24-h periodicity, either by comparison or correlation with the external time markers of sunrise (SR) and sunset (SS). The most obvious effect of late pregnancy, parturition and lactation was a distinct reduction of the activity level during the week of parturition and the next. Seasonal variations in the form of significant differences between the long-day summer half year and the short-day winter half year were established in the phase-angle differences of the morning activity peak to SR, in the evening activity peak and activity offset to SS, as well as in the activity time and the peak-to-peak interval, but not in the phase position of activity onset to SR or in the height of the morning and evening activity peak. These findings in combination with a high variability of the phase angle differences indicate that in A. geoffroyi, a relatively weak circadian component and strong masking direct effects of environmental factors are involved in the regulation of the daily activity rhythm.     

Natural Conditions Override Differences in Emergence Rhythm among Closely Related Drosophilids

Previous studies on adult emergence rhythm of Drosophila melanogaster (DM) done under semi-natural conditions have shown that emergence is correlated to daily changes in temperature, humidity and light at dawn. Recently we showed that under laboratory conditions D. ananassae (DA), a closely related species of DM exhibits patterns in its activity/rest rhythm distinct from the latter. Here, we report the results of a study aimed at examining whether this difference in activity/rest rhythm among species extends to other circadian behaviours such as the adult emergence rhythm under a more natural environment with multiple cyclic time cues. We monitored the adult emergence rhythm of recently wild-caught DM and DA populations in parallel with those of a related species D. malerkotliana (DK), both in the laboratory and under semi-natural conditions. We find that although DM, DK and DA showed marked difference from one another under laboratory conditions, such differences were not detectable in the emergence behaviour of these three species under semi-natural conditions, and that they respond very similarly to seasonal changes in the environment. The results suggest that seasonal changes in temperature and humidity contribute largely to the variation in adult emergence waveform in terms of gate width, phase and amplitude of the peak and day-to-day variance in the timing of the emergence peak. In all three species, seasons with cooler and wetter conditions make the rhythm less tightly gated, with low amplitude peak and high day-to-day variation in timing of the peak of emergence. We show that in nature the emergence rhythm of DM, DK and DA is strongly influenced by environmental factors such that in a given season all of them exhibit similar time course and waveform and that with the changing season, they all modify their emergence patterns in a similar manner.     

THE BEHAVIOUR OF THE WILLOW WARBLER PHYLLOSCOPUS TROCHILUS IN CONTINUOUS DAYLIGHT

Willow Warblers Phylloscopus trochilus at Wytham, Oxford (51° 46' N., 1° 15' W.), start to sing around sunrise, in late April to early June. In the continuous daylight at Neiden, East Finnmark (69° 42' N., 29° 25' E.), they do not sing all the time, but in late June to late July start singing between 22.45 and 01.45 hrs., the start becoming progressively later as the season draws on. At Neiden, nest visits start some time after song (01.00–02.30), and this is probably also true at Wytham.
The influence of environmental variables on the starts of the two cycles is discussed. It is concluded that song starts at Wytham when light intensity has risen above a certain threshold level, and at Neiden, when it has reached a minimum value or has started to rise again. It is suggested that the Neiden birds react to the closest approximation of their light intensity cycle to that at dawn further south. The end of daily song at Neiden may be controlled by internal factors. It is suggested that the start of the Neiden nest-visit cycle is controlled by light intensity, but modified by temperature effects. The possibility that the environment affects the bird indirectly, via its brood or food, can be ruled out. The functional significance of the retention of activity lulls in the continuous daylight of the Arctic summer is discussed.
The daily song-starts of the Garden Warbler Sylvia borin , Brambling Fringilla montifringilla , and Yellowhammer Emberiza citrinella , follow the same pattern as those of the Willow Warbler. The Redwing Turdus musicus often has no daily song lull; its nest-visit lull begins and ends well before midnight. The significance of this is discussed.     

Activity patterns at the Arctic Circle: nocturnal eagle owls and interspecific interactions during continuous midsummer daylight

Circadian rhythms result from adaptations to biotic and abiotic environmental conditions that cycle through the day, such as light, temperature, or temporal overlap between interacting species. At high latitudes, close to or beyond the polar circles, uninterrupted midsummer daylight may pose a challenge to the circadian rhythms of otherwise nocturnal species, such as eagle owls Bubo bubo. By non‐invasive field methods, we studied eagle owl activity in light of their interactions with their main prey the water vole Arvicola amphibius, and their competitor the white‐tailed eagle Haliaeetus albicilla during continuous midsummer daylight on open, treeless islands in coastal northern Norway. We evaluated circadian rhythms, temporal overlap, exposure, and spatial distribution. The owls maintained a nocturnal activity pattern, possibly because slightly dimmer light around midnight offered favourable hunting conditions. The eagles were active throughout the 24‐h period as opposed to the strictly diurnal rhythm reported elsewhere, thus increasing temporal overlap and the potential for interference competition between the two avian predators. This may indicate an asymmetry, with the owls facing the highest cost of interference competition. The presence of eagles combined with constant daylight in this open landscape may make the owls vulnerable to interspecific aggression, and contrary to the available literature, eagle owls rarely exposed themselves visually during territorial calls, possibly to avoid detection by eagles. We found indications of spatial segregation between owls and eagles reflecting differences in main prey, possibly in combination with habitat‐mediated avoidance. Eagle owl vocal activity peaked in the evening before a nocturnal peak in visual observations, when owls were active hunting, consistent with the hypothesis of a dusk chorus in nocturnal bird species. The owls may have had to trade‐off between calling and foraging during the few hours around midnight when slightly dimmer light reduced the detection risk while also providing better hunting conditions     

Circadian Variation in Phototactic Behaviour of Walking Indian Catfish, Clarias batrachus

Temporal organization in phototactic rhythm in male Clarias batrachus was studied in a choice chamber consisting of a photic zone and an aphotic zone. Results indicate that most of the fishes exhibited a statistically significant 24-h variation in the frequency of visits to the photic zone with a peak frequency at late night or early morning. Cosinor analysis of phototactic index (PI) values also yielded comparable results. This behaviour of C. batrachus shows similarity with its other activities, like air gulping, surfacing, foraging or predation that occur chiefly at night. The light intensity at the surface of a natural water body is more than at the bottom, even at night. Therefore, the peak phototactic activity observed in C. batrachus in the night or early morning could be attributed to its daily migration from the bottom to the surface of the water bodies. The results of the present study might be useful in aquaculture practice and inland fishing.     

濒危植物银杉幼树对生长光强的季节性光合响应

银杉(Cathayaargyrophylla)是我国松科中特有的单种属植物,被认为处于濒危状态。在对银杉群落多年调查研究的基础上,针对银杉幼树生长过程对光强的需求特性,我们开展了银杉幼树对光的适应性研究。试验在人工培育的银杉苗圃地,采用遮荫的方法设置不同的光环境处理(100%、45%和3%自然光强),利用气体交换技术和叶绿素荧光技术测定了3种光强下银杉叶片光合生理指标的变化,探讨了不同光环境下银杉幼树光合能力在夏季和冬季的变化及其对生长光强的响应等。结果表明:在夏季银杉生长旺盛时期,遮荫导致叶片最大光合速率(Pnmax)、羧化效率(CE)下降,但不同叶龄叶片的下降幅度不同。随生长光强的下降,银杉幼树的光补偿点(LCP)和光饱和点(LSP)有所降低,但全晴天时,低光强(3%自然光强)条件下实际的光辐射量高于当年生叶片光补偿点的累积时间约6h,而且与光饱和的区域相差极大,造成全天碳同化量低,同化物累积少,严重影响了银杉幼树的正常生长。在不同处理中全光强条件下银杉幼树长势最好,45%光强条件下幼树生长减慢。冬季银杉最大光合速率(Pnmax)、羧化效率(CE)值均低于夏季,光补偿点(LCP)和光饱和点(LSP)也较夏季降低。全光照条件下无论是当年生叶片和一年生叶片,在冬季均出现了轻微光抑制现象,适度遮荫有利于银杉抵御冬季光抑制。无论在遮荫或不遮荫条件下,冬季银杉叶片将所吸收的相对过剩光能通过非辐射途径耗散出去,表现出一种光保护策略。     

THE EFFECT OF SUCROSE SPRAYING ON SYMPTOMS CAUSED BY BEET YELLOWS VIRUS IN SUGAR BEET

When leaves of sugar-beet plants infected with beet yellows virus were sprayed daily with 10% sucrose solution, yellowing symptoms were intensified. When glasshouse plants were shaded so that the light intensity was reduced to less than half of full daylight, yellowing symptoms were suppressed more completely on un-sprayed than on sprayed plants. Spraying with 2–5 % sucrose solution had similar, but slightly smaller effects.
Spraying with sucrose solution increased the carbohydrate content of the leaves, and the effects on symptom intensity and carbohydrate content were closely correlated. The regression coefficients of symptom score on total sugar content were nearly the same for shaded and unshaded plants. As the severity of symptoms was increased by supplying carbohydrate without change in the light conditions, it is concluded that light intensity affects symptom expression by varying the carbohydrate content of the leaves through its influence on photosynthesis.
Sucrose spraying increased the yield of roots of healthy and infected plants, and most of the increase was sucrose. This shows that sprayed sugar was translocated to the roots from the leaves of both healthy and infected plants.
Measurements of changes in carbohydrate content between evening and morning samplings confirmed that movement of carbohydrate out of infected leaves is not stopped by infection.     

Circadian effects of light no brighter than moonlight

In mammals, light entrains endogenous circadian pacemakers by inducing daily phase shifts via a photoreceptor mechanism recently discovered in retinal ganglion cells. Light that is comparable in intensity to moonlight is generally ineffective at inducing phase shifts or suppressing melatonin secretion, which has prompted the view that circadian photic sensitivity has been titrated so that the central pacemaker is unaffected by natural nighttime illumination. However, the authors have shown in several different entrainment paradigms that completely dark nights are not functionally equivalent to dimly lit nights, even when nighttime illumination is below putative thresholds for the circadian visual system. The present studies extend these findings. Dim illumination is shown here to be neither a strong zeitgeber, consistent with published fluence response curves, nor a potentiator of other zeitgebers. Nevertheless, dim light markedly alters the behavior of the free-running circadian pacemaker. Syrian hamsters were released from entrained conditions into constant darkness or dim narrowband green illumination (~0.01 lx, 1.3 x 10(-9) W/cm(2), peak lambda = 560 nm). Relative to complete darkness, constant dim light lengthened the period by ~0.3 h and altered the waveform of circadian rhythmicity. Among animals transferred from long day lengths (14 L:10 D) into constant conditions, dim illumination increased the duration of the active phase (alpha) by ~3 h relative to complete darkness. Short day entrainment (8 L:16 D) produced initially long alpha that increased further under constant dim light but decreased under complete darkness. In contrast, dim light pulses 2 h or longer produced effects on circadian phase and melatonin secretion that were small in magnitude. Furthermore, the amplitude of phase resetting to bright light and nonphotic stimuli was similar against dimly lit and dark backgrounds, indicating that the former does not directly amplify circadian inputs. Dim illumination markedly alters circadian waveform through effects on alpha, suggesting that dim light influences the coupling between oscillators theorized to program the beginning and end of subjective night. Physiological mechanisms responsible for conveying dim light stimuli to the pacemaker and implications for chronotherapeutics warrant further study.     

Effects of twilights on circadian entrainment patterns and reentrainment rates in squirrel monkeys

Entrainment patterns of the circadian rhythms of body temperature and locomotor activity were compared in 6 squirrel monkeys (Saimiri sciureus) exposed to daily illumination cycles with abrupt transitions between light and darkness (LD-rectangular) or with gradual dawn and dusk transitions simulating natural twilights at the equator (LD-twilight). Daytime light intensity was 500 lux, and the total amount of light emitted per day was the same in the two conditions. Mean daytime body temperature levels were stable in LD-rectangular but increased gradually in LD-twilight, reaching peak levels during the dusk twilight. Locomotor activity showed a similar pattern, but with an additional, secondary peak near the end of dawn. Activity duration was about 0.5 h longer in LD-twilight than in LD-rectangular, but the time of activity midpoint was similar in the two conditions. Reentrainment of the body temperature rhythm was faster following an 8-h advance of the LD cycle than following an 8-h delay, but did not differ significantly between the two LD conditions. These results provide no evidence that the inclusion of twilight transitions affected the strength of the LD Zeitgeber, and suggest that the observed differences in the daily patterns reflected direct effects of light intensity on locomotor activity and body temperature rather than an effect of twilights on circadian entrainment mechanisms.Abbreviation LD light-dark